System and method of measuring convection induced impedance gradients to determine liquid flow rates

ABSTRACT

A method and system for measuring the flow rate of a liquid or gas within a flow channel utilizing a centrally located excitation source and a plurality of sensor means. Said excitation means is comprised of a heating element coupled with an alternating current generator. Of the plurality of sensor means, at least one of said sensors is located in a position upstream of the excitation source location, and additionally a second of said plurality of sensors is located in a position downstream of the excitation source. Instantaneous fluid flow rate is calculated utilizing a high gain differential amplifier electrically coupled to said sensors, wherein the convectively induced inductive gradient of the flowing fluid is compared to the symmetrical zero flow induction gradient. Following such a comparison, a voltage signal proportional to the flow of fluid within the channel is derived.

TECHNICAL FIELD

The present invention relates generally to the measurement of liquidflow rates through a defined tube or channel, and specifically relatesto the continuous flow measurement of flows less than 50 μL/min.

BACKGROUND OF THE INVENTION

There exist many examples of flow measurement through a fixed channel ortube. The simplest of these techniques utilizes a time of flightmeasurement, thereby only measuring flow through a channel for a fixedtime period. Such sampling, therefore, results in finite flowmeasurements, incapable of accounting for dynamic variations in fluidflow during those periods outside of the time of flight measurementperiod.

More advanced techniques utilize an external excitation and measurementmeans. In exciting a fluid, a heating means is typically utilized,wherein such a heater is capable of delivering thermal energy to aclosed fluid passageway. Two or more thermocouple probes, locatedupstream and downstream of the heating source, are then use to measurethe local temperature differences at fixed points along the flowchannel. Utilizing these components, a temperature gradient for astagnant and flowing fluid can be obtained. Based upon this data, afinalized fluid flow may be calculated.

While the localized heating and temperature measurement of a fluid, asset forth above, is useful in a variety of industrial applications, theoverall flow measurement accuracy is limited by several system inherentsources of error. Firstly, under the aforementioned flow measurementtechnique, system accuracy is directly related to thermocouple accuracy.To accurately determine fluid flow, each thermocouple must be able todiscriminate between discrete differences in fluid temperatures at theassociated thermocouple position. In systems in which there exists alarge heat introduction in a quickly moving fluid, such temperaturedifferences are greatly apparent. In light of this, determining fluidflow to the required degree of accuracy is made simple. In a setting inwhich there exists a small fluid flow velocity, the upstream anddownstream temperature variations are greatly reduced. One such settingmay be seen in a High Performance Capillary Liquid Chromatographic(HPLC) setting, in which fluid flows of less than 1 μL/min are notuncommon. In light of such a decrease in temperature variation, itbecomes important to measure temperature to a much higher degree ofaccuracy. Such an increase in accuracy requires the use of asignificantly more expensive thermocouple device.

Furthermore, under a temperature based system, there exist additionalsources of system error which are not easily prevented. One such sourceof error lies in the thermal conduction of heat through the walls of thefluid channel. Such conductive heat transfer from heat source tothermocouple position results in the loss of flow measurement accuracy,as the thermocouple is no longer solely recording fluid temperature, butrather is under the influence of additional heat addition through thewalls of the flow channel. Convective losses of heat along the flowchannel exterior additionally contribute to the inherent systeminaccuracies. Should a user elect to use externally mountedthermocouples, which are seated along the external surface of the flowchannel, additional error is introduced as said external thermocouplesare merely recording the fluid boundary layer temperature, as opposed tothe interior fluid temperature at points away from the boundary layer.Numerous attempts have been made in the art to prevent these conductiveand convective losses, all of which result in increases in system costand complexity.

In light of the above, when operating in a low flow environmentutilizing thermocouples as sensor means, the use of an internalthermocouple element is important in providing the greatest degree offlow rate measurement accuracy. Such internal thermocouples are indirect physical contact with the flowing fluid and offer greatsensitivity and time response. Direct contact between an internaltemperature probe and fluid, however, results in potential contaminationof the fluid or of the temperature probe element. As the fluid is indirect contact with the internal probe, particulate matter suspended ina flowing fluid may contaminate the external temperature sensing regionof a temperature probe, thereby resulting in inaccurate measurements.Furthermore, in the presence of highly corrosive or chemically reactivefluids, contact between the internal temperature probe and these fluidsmay result in the break down of the internal temperature probe surface,thereby resulting in the introduction of contaminant into the flowingfluid.

Additionally, the use of direct contact thermocouples require theintroduction of numerous fittings, couplings and related alterations tothe flow channel to adequately introduce the temperature measuringprobes to the fluid flow. These extraneous additions introduce numeroussources of failure or leakage. In a high pressure operating environment,such as a HPLC setting, the addition of flanged or threaded couplings toa fluid channel requires skilled assembly of precision components. Suchfittings introduce several potential failure locales when compared to acontinuous, uninterrupted flow channel. Additionally, the introductionof these aforementioned temperature sensors into the fluid channelgreatly increases system costs and complexity.

Finally, in a liquid chromatography environment, the addition of theseaforementioned couplings, fittings and invasive temperature elements tothe fluid pathway greatly increase the “dead volume” of thechromatographic column. The term “dead volume” is used to describe theunknown volume which is trapped in these various fittings, couplings,and thermocouple regions of the flow channel. This fluid may be eitherstagnant or dynamic in nature and may unpredictable escape into thefluid channel thereby causing the shape of the fluid pulse to be alteredfrom the desired shape.

SUMMARY OF THE INVENTION

The present invention allows the user to measure fluid flow rate whilesimultaneously minimizing dead volume in a chromatographic environmentwherein small fluid flows are present. The minimizing of dead volumereduces the cycle time of gradients, guards against convective and eddycurrent mixing, and aids in providing highly reproducible results. Inattaining the foregoing and other objects, the present inventionprovides methods and apparatus for mounting an excitation source, aswell as a plurality of sensor elements, to the external surface of aflow channel. The excitation source is located either in direct contactor in close proximity to the external surface of an existing flowchannel. The excitation source is designed such that it may be mountedto a prexisting flow channel, thereby allowing retrofitting of existingsystems. The excitation source may take the form of an alternatingcurrent (AC) generator coupled with an integral heating means.Additionally, alternate forms of excitations sources may be substitutedas understood by those skilled in the art.

In addition to the excitation source, a plurality of sensor elements arelocated in positions upstream and downstream of the excitation source.Similar to the excitation source, these sensor elements are designedsuch that they may be installed on a prexisting flow channel withoutintrusive modifications. In light of this, dead volume within a flowchannel is not increased by adding unnecessary couplings, fittings orother forms of extraneous paraphernalia associated with the installationof a direct contact sensor element. This excitation source comprises aheating element, used in elevating the temperature of the fluid, as wellas an alternating current (AC) signal generator. This heater and ACgenerator are packaged together, and are located around the externalsurface of a flow channel. Following the addition of heat to the fluid,the fluid impedance will change. This change in impedance may bedetected by a plurality of sensor means located in positions bothupstream and downstream of the excitation source location. A preliminaryimpedance reading at upstream and downstream sensor locations under azero flow condition may be recorded thereby providing a baselinerepresentation of impedance conditions within the flow channel. Such abaseline representation will result in a symmetrical impedance gradientcentered at the point of excitation. Furthermore, additional impedancereadings from the aforementioned plurality of sensor means may berecorded under fluid flow conditions, thereby yielding a asymmetricalimpedance gradient around the central excitation element. Theseaforementioned sensor means are in electrical communication with acalculation means, wherein said calculation means comprises a high gaindifferential amplifier capable of receiving electrical information fromsaid sensors. This high gain differential amplifier yields an electricalvoltage proportional to the fluid flow rate within the flow channel.Under a zero flow condition, for example, this calculation means willyield a zero output voltage. Under a flowing fluid scenario, forexample, the output of this high gain differencing amplifier may berepresented by a positive voltage proportional to the fluid velocitywithin the channel. The flow rate measurement may be continuous innature and provides instantaneous values of flow rate within a flowchannel. In light of such an arrangement, continuous flow measurementmay be calculated which represent actual flow conditions within achannel at any instant in time. When coupled to a High Pressure LiquidChromatographic system (HPLC), the present invention may be utilized inproviding flow feedback information to the HPLC system operating in asub 50 μL/min environment, to thereby verify delivery performance of thepumping means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting the typical induction gradient for astagnant and flowing fluid contained within a flow channel following theaddition of heat at a fixed point.

FIG. 2 is a schematic of an embodiment of the present invention, inwhich contactless conductive pickups are employed in positions upstreamand downstream of an excitation source. Such contactless conductivepickups are capable of measuring fluid impedance, and are in electricalcommunication with a high gain differential amplifier.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2, wherein like parts are designated by like referencenumerals throughout, illustrate an example embodiment of a system andmethod suitable for measuring the flow of a liquid through a flowchannel. This system and method may be used alone or in combination witha High Pressure Liquid Chromatography device. Although the presentinvention is described with reference to the example embodimentsillustrated in the figures, it should be understood that manyalternative forms could embody the present invention. One of ordinaryskill in the art will additionally appreciate different ways to alterthe parameters of the embodiments disclosed, such as the heating means,size, or type of sensor means, in a manner still in keeping with thespirit and scope of the present invention.

Referring to FIG. 1, a static impedance distribution (2) is shownwherein thermal energy and an alternating current signal is introducedat a fixed excitation point (6), and fluid impedance is measured atfixed distances upstream (8) and downstream (10) of the point of thermalenergy introduction (6). Additionally, an impedance/distancedistribution in a flowing fluid environment (4) is depicted wherein theexcitation point (6) remains the same and inductance is measured at thesame fixed upstream position (8) and downstream position (10).

Referring to FIG. 2 of the present application, a flow channel (20) isdepicted which utilizes an excitation source comprising a heater element(26) and an alternating current signal generator (24). Said elements arein contact with the external wall of the flow channel, and therebyintroduce energy to the fluid (30) within the flow channel (20). Theseexcitation elements are not in direct physical contact with the fluid(30) contained within the flow channel (12) thereby preventingcontamination of the fluid or the heating element (26) or thealternating current signal generator (24). Additionally, a firstcontactless conductive impedance sensor (22), located at a pointupstream of the AC signal generator (24) and heat source (26) is taught.A second contactless conductive impedance sensor (28) is located at apoint downstream of the location of the excitation elements. Thisupstream sensor (22) and downstream sensor (28) are located at fixeddistances from the heat source (26) and AC current generating source(24). The upstream and downstream sensors are individually amplified byhigh gain amplifiers (32) (34) and are processed by a high gaindifferencing amplifier (36) thereby yielding a flow dependent outputvoltage signal.

1. A flow rate measuring meter, comprising: a flow channel with aninterior passage having an input end and an output end, wherein saidinput end is capable of receiving a liquid whose flow rate is to bemeasured; an external excitation source mounted to an external surfaceof an external wall of the flow channel for exciting the liquid; aplurality of sensor means for measuring a fluid impedance of liquidwithin the flow channel, wherein at least one of said sensor means islocated upstream of the excitation source and a second sensor means islocated in a position downstream of said excitation source, wherein saidplurality of sensor means comprises conductive pickups that do notcontact said liquid in said flow channel; and a calculation means incommunication with said plurality of sensor means, wherein output ofsaid calculation means is a flow rate dependent electrical signal. 2.The meter of claim 1, wherein said excitation source comprises a heatingmeans coupled to an alternating current generating conductor.
 3. Themeter of claim 1, wherein said excitation source is in close proximitywith the external wall of the flow channel.
 4. The meter of claim 1,wherein said excitation source intimately contacts the external wall ofthe flow channel without compromising the physical integrity of saidchannel.
 5. The meter of claim 1, wherein said excitation source may belocated around the external wall of a pre-existing flow channel.
 6. Theplurality of sensor means of claim 1, wherein said sensor meansintimately contact the external wall of the flow channel withoutcompromising the physical integrity of the flow channel.
 7. Theplurality of sensor means of claim 1, wherein said sensor means may belocated around the external wall of a pre-existing flow channel.
 8. Themeter of claim 1, wherein said sensor means are located such that theyare external to the fluid contained within the interior passage of theflow channel, yet are in conductive contact with the external surface ofthe flow channel.
 9. The calculation means of claim 1, wherein saidcalculation means comprises a high gain differencing amplifier inelectrical communication with said plurality of sensor means such thatthe output of this high gain differencing amplifier is a voltage signalproportional to the flow rate within the flow channel.
 10. A method formeasuring flow rate comprising the steps of: introducing a flowing fluidto the internal passage of a flow channel; exciting the fluid utilizingat least one external excitation source mounted to an external surfaceof an external wall the flow channel; measuring an amount of additionalenergy added to the flowing fluid at a first position upstream of theexcitation source and at a second position downstream of the excitationsource by sensing fluid impedance, wherein the step of measuring anamount of additional energy comprises sensing fluid impedance utilizinga plurality of sensor means in conductive contact with the externalsurface of the flow channel; and calculating the instant flow rate ofthe flowing fluid based upon the asymmetrical fluid impedance gradientof the flowing fluid as determined by said step of measuring.
 11. Themethod of claim 10, wherein the step of exciting the fluid comprisesutilizing a thermal heating element and an alternating currentgenerating conductor.
 12. The method of claim 11, wherein the step ofcalculating the asymmetrical fluid impedance gradient further comprisesthe step recording fluid impedance under a flowing fluid condition. 13.The method of claim 10, further comprising the step of calculating saidasymmetrical fluid impedance gradient by recording fluid impedance undera zero flow condition.
 14. The method of claim 10, further comprisingthe step of calculating continuous fluid flow through a flow channelutilizing a high gain differencing amplifier to produce a voltage signalrepresentative of instantaneous fluid flow conditions based on said stepof measuring.
 15. The method of claim 14, comprising the steps ofcomparing a representative voltage signal for zero flow conditions withthat of the voltage signal under flowing fluid conditions, therebyyielding a voltage signal representative of instantaneous fluid flow.16. A high performance liquid chromatography (HPLC) system, including aflow rate measuring meter, the improvement comprising: a flow channel;an external excitation source mounted to an external surface of saidflow channel for exciting a liquid in the flow channel; a plurality ofsensor means wherein at least one of said plurality is located in aposition upstream of said excitation source and a second of saidplurality is located in a position downstream of said excitation source,wherein said plurality of sensor means comprises impedance sensors thatdo not contact said liquid in said flow channel; and a calculation meansin electrical communication with said plurality of sensor means forreceiving signals from said plurality of sensor means indicative of afluid impedance of a fluid within the flow channel, wherein the outputof said calculation means is a voltage signal proportional to theinstant flow rate of the fluid within the flow channel.
 17. The highperformance liquid chromatography system of claim 16, wherein said flowchannel is designed to minimize dead volume.
 18. The high performanceliquid chromatography system of claim 16, wherein said flow measuringmeter is used as an element within a flow sensing feedback means forverifying delivery of a fluid with a flow rate of less than 50 μL/min.19. The high performance liquid chromatography system of claim 16,wherein said flow meter is capable of continuous flow measurements of aflowing fluid within a flow channel.
 20. The high performance liquidchromatography system of claim 16, wherein said external excitationsource comprises a heating element and an alternating current generatingelement.